1. Field of the Invention
The present invention relates to vacuum insulated containers, and more particularly to such containers adapted for use in shipping products, such as products which must be kept at very low temperatures for relatively long periods of time.
One of the common uses of insulated cargo containers is for the shipment of frozen food stuff. Such containers can be designed to produce temperatures below 0.degree. F. However, with age, quite commonly deterioration of the insulation and also of the refrigeration equipment occurs, this resulting in reduction of the subzero capabilities of such containers. Even though the operators who use such insulated cargo containers attempt to maintain a high quality of service, the cost of doing so has been continuously increasing over the years. Further, in many instances, maintaining the temperature of food stuffs at approximately 0.degree. F. is not optimum for food quality maintenance.
It has been known for years that the fast freezing of food stuffs, such as fruits, vegetables, fish and other commodities, using cryogenic fluids such as liquid nitrogen can result in a superior market product. While these techniques have been used and automated equipment has been developed to perform the freezing operation, the problem of shipping at quite low temperatures (i.e. near -80.degree. F.) has been a very difficult one to solve. Thus, though the near 0.degree. F. shipping temperatures are not optimum for food quality maintenance, for the large part, shipping containers having the capability to be used only for shipment at temperatures at about 0.degree. F. are the current state of the art.
It has long been known that excellent insulating capability can be obtained by providing a vacuum between two members, a common device utilizing this principle being the vacuum flask. Such a flask is made up of inner and outer walls which are spaced from one another, with a vacuum being provided in the space between the two walls. Quite commonly the two walls are formed as concentric cylindrical side wall sections, with the ends of the cylinders being closed by concentric hemispherical sections. An opening is provided through one of the end hemispherical sections.
However, the walls of the vacuum flask are subjected to rather substantial forces. With atmospheric pressure being approximately 15 pounds per square inch (psi) at sea level, the outside wall of a 3 inch diameter by 12 inch long standard vacuum bottle is subjected to a total lateral force of as much as 540 pounds. The internal wall of the flask does not require as heavy a wall, since the internal forces are directed radially outwardly, so that the material forming the inner wall is in tension, with there being no buckling tendency. However, the outer wall experiences what can be described as a crushing force, and the outer wall must be structurally stronger to withstand the forces which would tend to buckle the outer wall.
Because of the structural problems of providing a vacuum insulating container, in many instances the thought of using the evacuated area as insulation is abandoned, and thick high quality insulation is used. However, to maintain quite low temperatures for long periods of time, even the use of quite thick, high quality insulation is not satisfactory.
Another consideration is that in any shipping container, the volume occupied by the container is an important consideration. Desirably, the total volume occupied by the container should not be too much greater than the volume of the product contained. Further, it is desirable that the configuration of the shipping container be such so that the loading of the containers into, for example, a truck or a freight car, can be accomplished as economically as possible, with the optimum use of space.